An autoradiographic study of the mouse olfactory epithelium: evidence for long-lived receptors.

In order to try to determine whether differentiated olfactory receptors turn over (die and are replaced by newly differentiated cells) during adult life, mice were injected with a single dose of 3H-thymidine at either 2 or 4 months of age and allowed to survive for up to 12 months; they were caged in a laminar flow unit to prevent rhinitis. Counts of labeled receptor cells detected autoradiographically after injection at 2 months of age revealed that, following an initial decrease from 1 to 3 months of survival, numbers of labeled cells remained approximately constant, at least up to 12 months of survival. Cells still labeled at 12 months of survival were confirmed as receptor cells by electron microscopic examination of reembedded sections. The hypothesis is suggested that in the absence of disease-related destruction of the olfactory epithelium, most or all receptor cell turnover represents newly formed cells that fail to establish synapses with the olfactory bulb; fully differentiated receptor cells may be quite long-lived.

Development of retinal amacrine cells in the mouse embryo: evidence for two modes of formation.

Developing amacrine and ganglion cells have been graphically reconstructed from a series of 567 consecutive thin sections of the E17 mouse retina on the first day when an obvious inner plexiform layer (IPL) is present and 2 days later than for our previous study of amacrine cell formation at E15 (Hinds and Hinds ('78). At E17 amacrine cells of the neuroblastic layer (normally placed amacrine cells), unlike those at E15, appear to develop directly from ventricular cells; intermediate elements are bipolar-shaped cells with terminal arborization in the IPL. On the other hand, the development of displaced amacrine cells and some normally placed amacrine cells at E17 appears to closely resemble that described for all amacrine cells at E15: derivation from "ganglion cells" by loss of the axon and transformation of the cell. Three lines of evidence support this conclusion. (1) Cells have been found that resemble ganglion cells except that they have only an apparent axon remnant and have somata restricted to the IPL and the immediately adjacent portion of the ganglion cell layer (GCL); amacrine cells transitional between these cells and the smaller and darker, normally placed amacrine cells also occur in the IPL. (2) Axons of two ganglion cells have been found which appeared to be in the process of breaking up and degenerating. (3) The fraction of anaxonic cells with somata in the GCL (two out of 79, or 3%) or in the GCL plus IPL (ten out of 88 or 11%) is too small to account easily for the large fraction (probably at least 45%) of displaced amacrine cells found in the adult, even with conservative assumptions (P less than 0.05). A mathematical model suggests that approximately 40% of the ganglion cells present at E17 will lose their axon, and of these around half will migrate to the neuroblastic layer, while the other half will become displaced amacrine cells. The results suggest a natural explanation for the recent finding that wide field amacrine cells are found with somata on both sides of the IPL, while narrow field amacrine cells are never displaced: the former may be derived from ganglion cells by loss of the axon, while the latter may be formed directly from ventricular cells.

Differentiation of photoreceptors and horizontal cells in the embryonic mouse retina: an electron microscopic, serial section analysis.

The early differentiation of photoreceptors and horizontal cells in the mouse retina has been studied with serial thin sections and reconstructions in embryos on the fifteenth and seventeenth days of gestation (E15 and E17). The following developmental sequences have been inferred. At E15 photoreceptors develop from ventricular cells when a long vitreal process fails to develop following mitosis, and the end of the ventricular process forms a bulbous enlargement (the future inner segment) which contains a pair of centrioles and a cilium and extends into the optic ventricle. This future inner segment is considerably larger at E17, but otherwise the photoreceptors resemble those seen at E15. At E15 horizontal cells develop from ventricular cells when a long vitreal process fails to develop following mitosis, and the end of the ventricular process detaches from the junctional complex at the ventricular surface. By E17 future horizontal cells are located in the middle of the ventricular layer (neuroblastic layer) and have developed from bipolar shaped cells into cells with multiple branching processes, predominantly radially arranged but rarely with a more tangential orientation. These relatively advanced cells at E17 resemble closely the earliest stage of horizontal cell formation described previously in silver studies by Cajal. A scheme is proposed which explains the initial differentiation of several of the major cell types in the retina in terms of two key features: whether or not the call remains attached to the junctional complex and whether or not a vitreal process grows into the ganglion cell layer. By independent variations in these two features, four classes of cells are produced that, by virtue of their differing environments, differentiate into four cell types: ganglion (and amacrine) cells, horizontal cells, photoreceptors, and Müller cells.

Synapse formation in the mouse olfactory bulb. I. Quantitative studies.

A quantitative study of synapse formation in the mouse olfactory bulb has been carried out using serial sections. Volumetric synaptic density as well as absolute number of synapses per olfactory bulb for eight distinct synaptic types have been determined at 15 different ages, from the beginning of synapse formation at embryonic day 14 (E14) to postnatal day 44 (P44). Synapses are first found in appreciable numbers at E15 when both axo-dendritic and a few dendro-dendritic synapses occur in the presumptive glomerular layer. Initial synapse formation correlates closely with the reorientation of mitral cells from a primitive tangenital to a definitive radial direction. Synapse formation by mitral cell dendrites occurs after mitral cell axons have grown into the future olfactory cortical areas, either simultaneous with or before synapse formation by these axons. Virtually all synaptic types detected in adults have been found on the day of birth, consistent with the idea that olfaction is an important sensory modality for newborn mice. Volumeric density of a given synaptic type generally increases 50--100 times during development while the absolute number increases about 1,000 times. Synapses in glomeruli develop more precociously than those in the time of origin and differentiation of the principal postsynaptic elements of these two divisions (mitral cells and internal granule cells). Correlation of the time of synapse formation of various synaptic types with their putative excitary or inhibitory role determined in adult studies suggests that excitatory synapses generally form somewhat earlier, although throughout nearly all of synaptic development, both excitatory and inhibitory synapses are present. Reciprocal dendro-dendritic synapses in the external plexiform layer appear to have a special mode of formation. It is suggested that a granule-to-mitral dendro-dendritic synapse only forms next to an already existing mitral-to-granule synapse on the same gemmule.

Synapse formation in the mouse olfactory bulb. II. Morphogenesis.

Serial thin sections of the mouse olfactory bulb from the fourteenth day of gestation (E14) to postnatal to 44 (P44) have been examined in order to study morphogenesis of individual synaptic junctions. At the initiation of synapse formation structures are found that resemble postsynaptic densities but are facing extracellular space or unmodified processes. Transition forms between these isolated postsynaptic densities and undoubted synapses have been found. These observations as well as quantitative studies support the hypothesis that isolated postsynaptic densities can form independently and be converted to synapses when a presynaptic specialization develops opposite them. Detailed studies of olfactory axodendritic synaptogenesis throughout the entire developmental period suggests strongly that these asymmetrical synapses pass through an immature symmetrical phase: (1) symmetrical olfactory axodendritic synapses are found in significantly higher concentration on axonal and dendritic growth cones than on more common processes; (2) the number of symmetrical synapses is correlated with the rate of formation of new synapses determined previously. The time for a recognizable symmetrical synapse to be transformed into a recognizable asymmetrical one has been calculated to be 9--10 hours. A scheme of synapse formation in the CNS has been proposed in which a post-synaptic structure forms independently followed by aggregation of pre-existing presynaptic components into a presynaptic specialization. Different morphogenetic sequences of synapse formation from region to region are attributed simply to different relative rates in the development of the postsynaptic density and the presynaptic specialization.